A pneumatical blasting machine is disclosed in DE-A No. 225,021. FIGS. 1 and 2 of the disclosure show embodiments of such an apparatus in which an air stream is either used in the blast-air duct as an additional transport means or an allowance is made for it. In the first embodiment, the air stream, runs in the screw shaft itself causing a strong entraining effect at the end of the spiral, which leads to disturbances and irregularity in starting up the apparatus. In the second embodiment, the shaft of the screw is also an air carrying means. It contains additional side channels which run into the screw conveyor tube and entrain the blasting agent. In this embodiment as well turbulence, air shocks and other such phenomena will result creating irregularities in the operation of the device, especially when the apparatus is started up and shut down.
Conveyor screws which operate in a conveyor tube are generally known and installed in many technical apparatus. Precise studies have been made of their conveying characteristics, screw pitch, output, and the like, so that either by a few experiments or on the basis of manufacturer's instructions and tables, it is possible to establish the accurate output of a conveyor screw with reference to a particular blasting agent. See, for example, the proposals for calculations made by FISCHER in HUETTE II B.
It was surprisingly found that the device according to the present invention solved a difficult problem of achieving constant and reproducible blasting agent flow rates by, in part, providing a conveyor screw with a stepless rotatory speed which can be finely adjustable to achieve precise rates of feed with exact reproduction. In addition, any pressure drop that may exist between the air pressure in the discharge area of the screw conveyor tube and the air pressure in the blast-air duct can be eliminated. This means of achieving air pressure equalization between the tube discharge and the blast air duct is not taught in the prior art apparatus, since compressed air in the known devices generally provides for an overpressure in the hopper and serves to force out the blasting agent. The hopper pressure must therefore generally be higher than the back pressure in the blasting agent feed in the know devices.
Lastly, it is also important to prevent undesired suction or pressure effects from occurring in pneumatic blasting devices, which result from Bernouilli's law This law of aerodynamics states that in a flow of incompressible fluid the sum of the static pressure and the dynamic pressure along a streamline is constant, if gravity and frictional effects are disregarded. It thus follows that where there is a velocity increase in a fluid flow, there must be a corresponding pressure decrease.
Particularly in the case of so-called suction jets, an injector nozzle is provided in the blasting agent delivery hose which produces a vacuum. Thus, shocks, gas bubbles and the like can occur, resulting in the unbalancing of the blasting agent stream. An apparatus of the kind described, in DE-A No. 160,779 does not disclose a means for eliminating such disturbances.